TITLE: GCN CIRCULAR NUMBER: 7800 SUBJECT: GRB 080604: Swift detection of a burst with an optical afterglow DATE: 08/06/04 07:38:24 GMT FROM: David Palmer at LANL V. La Parola (INAF-IASFPA), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), C. Guidorzi (INAF-OAB), S. T. Holland (CRESST/USRA/GSFC), N. P. M. Kuin (MSSL), J. Mao (INAF-OAB), P. T. O'Brien (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), D. Perez (U Leicester), B. Sbarufatti (INAF-IASFPA), R. L. C. Starling (U Leicester), D. E. Vanden Berk (PSU) and H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team: At 07:27:01 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 080604 (trigger=313116). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 236.970, +20.555 which is RA(J2000) = 15h 47m 53s Dec(J2000) = +20d 33' 19" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a broad single peak structure with a duration of about 70 sec. The peak count rate was ~300 counts/sec (15-350 keV), at ~10 sec after the trigger. The XRT began observing the field at 07:29:00.4 UT, 119.3 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 236.9648, +20.5572 which is equivalent to: RA(J2000) = 15h 47m 51.5s Dec(J2000) = +20d 33' 25.9" with an uncertainty of 5.4 arcseconds (radius, 90% containment). This location is 19 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy. UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 128 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 15:47:51.70 = 236.9654 DEC(J2000) = +20:33:28.1 = 20.5578 with a 1-sigma error radius of about 0.5 arc sec. This position is 3.4 arc sec. from the center of the XRT error circle. The estimated magnitude is 20.1 with a 1-sigma error of about 0.5 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.05. Burst Advocate for this burst is V. La Parola (laparola AT ifc.inaf.it). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/too.html.)